1. Field of the Invention
The present invention relates to an apparatus for heat treating an optical ceramic material, a method for heat treating an optical ceramic material, a method for heat treating synthetic silica glass, a method for producing an optical system, and a method for producing an exposure apparatus.
2. Description of Related Art
As shorter wavelengths are increasingly used as a light source of an exposure apparatus, very high optical transmittance in such shorter wavelength light is needed for optical ceramic materials such as synthetic silica glass, calcium fluoride, barium fluoride and the like that are used in optical components constituting the optical system in the exposure apparatus. Optical ceramic materials that require such high optical transmittance is produced from a chemically synthesized raw material having high purity.
However, in the inside of the produced optical ceramic material, there remain various stresses derived from thermal history when it is produced. Accordingly, in order to reduce the stresses, a heat treatment called annealing treatment is performed to improve the homogeneity of refractive index by decreasing residual stresses and to decrease birefringence of the produced optical ceramic material (see Japanese Laid-open Patent Publication No. 2005-22921).